The FX6300 and 7870 will be much, much better for gaming. The i5 combo is no match. Also socket 1155 becomes obsolete in a month or two, whereas AM3+ has atleast another line of processors before it's dropped.
The PCIE 3.0 compatibility is such a non-issue. No graphics card approaches anywhere near the full PCIE 2.0 bandwidth, let alone that of PCIE 3.0.
If you want to build a gaming computer, never spend more on the processor than the graphics card. The gaming performance of the 7870 LE system will be much better, simply because it's the graphics which define gaming performance, not the processor in 90% of games.
The best option would be the i5 + 7870 LE obviously. If your budget can't stretch to that though, get either an i3 + 7870 LE or FX 6300 + 7870 LE.
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